Micromechanical tuning fork gyroscopes such as that shown in FIG. 1 are known. The tuning fork gyroscope includes silicon proof masses which are suspended by support flexures above a glass substrate, and comb electrodes which are used for oscillating the proof masses. Metallic sense electrodes are disposed on the glass substrate below the proof masses for detecting Coriolis motion by indicating out-of-plane capacitance changes. Because the tuning fork gyroscope operates in this manner, it is desirable that the amplitude of the oscillation be held at a predetermined constant in order to provide a more accurate output indicating rate.
The amplitude of the oscillating motor of tuning fork gyroscopes is typically controlled by a conventional servo loop connected to a single capacitive in-plane pick-off ("center electrode"). In this technique motor position is converted to a proportional voltage by measuring charge variation on the center electrode which is biased with a DC voltage. The resulting motor position signal is amplified and detected by a full wave rectifier. The rectifier output is then filtered, and the filtered voltage compared against a reference voltage; the difference forming an error voltage. This error voltage is then used to control the motor drive amplitude using a loop controller to adjust motor amplitude to a predetermined constant. However, this particular technique has a potential drawback.
The conventional servo loop technique may have some instability in the center electrode. The sensitivity of the DC biased center electrode varies slowly over time due to a spurious charge accumulation on the glass substrate beneath the center electrode. As this charge accumulates on the glass, the sensitivity of the center electrode is modified. In response, the loop controller calls for modified drive force to compensate. The result is a motor amplitude transient wherein amplitude changes over time as substrate charge accumulates. This will result in lower accuracy than is otherwise possible because of the relationship between amplitude and Coriolis force in the system.